MD Study of Stokes Shifts in Ionic Liquids: Temperature Dependence

Effects of temperature on Stokes shifts, solvation structure, and dynamics in ionic liquids EMI(+)Tf2N(-), EMI(+)PF6(-), and BMI(+)PF6(-) (EMI(+) = 1-ethyl-3-methylimidazolium, BMI(+) = 1-butyl-3-methylimidazolium, Tf2N(-) = bis(trifluoromethylsulfonyl)imide, and PF6(-) = hexafluorophosphate) are investigated via molecular dynamics (MD) computer simulations in the temperature range 350 K ≤ T ≤ 500 K. Two different types of solutes are considered: a simple model diatomic solute and realistic coumarin 153, both of which are characterized by more polar S1 and less polar S0 states. In all three ionic liquids studied, the Stokes shift tends to decrease with increasing temperature. For coumarin 153, as T increases, the Franck-Condon energy for steady-state absorption decreases, whereas that for steady-state emission increases. Our findings indicate that the effective polarity of ionic liquids decreases as T increases. Their solvation dynamics are characterized by an ultrafast initial decay in the subpicosecond time scale, followed by slow dissipative relaxation, regardless of temperature. For both solutes, the solvent frequency that quantifies initial ultrafast dynamics shows little temperature dependence. By contrast, the long-time dissipative dynamics become significantly faster with rising T. Variations of solvation structure with temperature and their connection to Stokes shift and solvation dynamics are briefly examined.